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NATURALIST [Vol. XLI 



contraction of the protoplasm above the cilium carries its base up- 

 ward, and cramps the cihum against the overhanging edge of the 

 groove until the lower portion of the cilium is bent into an S-shaped 

 curve (Fig. 1, c and d). The contraction continuing, the cilium is 

 forced past the edge of the groove and flies out and back with a 

 very rapid stroke — the efl'ective stroke of the cilium (Fig. 1, e). 

 It is carried by the force of its stroke far beyond its position of 

 rest to which its elasticity brings it back in position for another 

 stroke. There seems no doubt that these cilia are elastic rods 

 (Fig. 2, g) which are moved by the contraction of the protoplasm at 

 their bases. The cuticula around the base of the cilium rises and 

 falls with the movement of the cilium as though it formed a plate 

 into which the cilium is set. Consequently it appears that the 

 contractile fibrillae of the protoplasm are inserted in the cuticula 

 and not directly in the base of the cilium. We do not know what 

 part the basal body plays in this movement but we believe that 

 it forms a pivot upon which the cilium turns somewhat as an 

 echinoderm spine turns upon its base. 



The cramping and subsequent escape of the cilium account for 

 the rapidity and force of the effective stroke and also explain the well 

 known fact that the cilia of rotifers and veligers always seem to move 

 only in the opposite direction to that which is necessary, since the 

 effective stroke is too rapi<l to he visible. We do not believe that 

 the groove is a commoi\ ciliary structure, — on the contrary it is 

 probably present only in cilia like those mentioned, whose effective 

 stroke is invisible. However, we should call attention to the pits, 

 collars, and ridges at the bases of the flagella and cilia of Protozoa, 

 Porifera, and spermatozoa to which as yet no function has !)een 



